U.S. Pat. No. 3,928,196 discloses the use of copolymers of 2-acrylamido-2-methylpropylsulfonic acid and acrylic acid as scale inhibitors.
U.S. Pat. No. 4,640,793 discloses the use of admixtures containing carboxylic acid/sulfonic acid polymers and phosphonates as scale and corrosion inhibitors.
U.S. Pat. No. 4,618,448 discloses the use of polymers comprising an unsaturated carboxylic acid, an unsaturated sulfonic acid and an unsaturated polyalkylene oxide as scale inhibitors.
Japanese No. 57-084794 discloses the use of copolymers of acrylic acid and allyl polyethylene glycol as scale inhibitors.
European patent application No. 84301450.7 discloses carboxylic acid/sulfonic acid copolymers in combination with organic phosphonates as scale inhibitors.
U.S. Pat. No. 4,510,059 discloses the use of carboxylic functional polyampholytes to reduce silica deposits in aqueous systems.
U.S. Pat. No. 4,532,047 discloses a method of inhibiting amorphous silica scale formation using polypolar organic compounds and borate ion sources.
U.S. Pat. No. 4,584,104 discloses a method of inhibiting amorphous silica scale formation using a source of orthoborate ions.
Silica/silicate deposition in aqueous systems, for example boilers, cooling towers and systems containing hypersaline geothermal brines, is a continuing problem. Traditionally, deposition has been controlled by softening the makeup water to the system being treated, by blowdown, or by both. If deposition occurs, mechanical removal or washing with ammonium fluoride or hydrofluoric acid is generally the method of control. Obviously, mechanical or chemical cleaning causes down time and increased energy and labor costs.
pH affects the ionization of silanol groups and, therefore, affects the polymerization rate. Silica first forms, then three dimensional networks form. Eventually, colloidal particles grow through condensation. At pH 7, nuclei formation and particle growth is very rapid. The pH of cooling water is generally 6.0 to 8.5 and the water temperature is generally about 30.degree. to 70.degree. C. The pH of geothermal brines is generally 4.0 to 6.0 and the brine temperatures is generally about 100.degree. to 210.degree. C.
It is known to use cationic polymers or cationic surfactants as silica scale inhibitors in hypersaline geothermal brines (Harrar, J. E. et al, "Final Report on Tests of Proprietary Chemical Additives as Anti-scalants for Hypersaline Geothermal Brine", January 1980, Lawrence Livermore Laboratory, Harrar, J. E. et al, "On-Line Tests of Organic Additives for the Inhibition of the Precipitation of Silica from Hypersaline Geothermal Brine IV, Final tests of Candidate Additives", February 1980. Lawrence Livermore Laboratories; and Harrar, J. E. et al, "Studies of Scale Formation and Scale Inhibitors at the Salton Sea Geothermal Field", Corrosion/80. Paper No. 225, International Corrosion Forum, devoted exclusively to the Protection and Performance of Materials, Mar. 3-7, 1980. Chicago, IL).
The inventors have discovered a method for controlling the deposition of silica and silicates in an aqueous system using a phosphonate selected from the group consisting of hexamethylene diamine tetra(methylene phosphonic acid), diethylene triamine penta(methylenephosphonic acid), salts of these compounds and combinations thereof, alone or optionally in combination with a water soluble polymer prepared from at least one carboxylic acid moiety, at least one sulfonic acid moiety and, optionally, at least one polyalkylene oxide moiety. While the designated phosphonates alone are effective inhibitors, the designated polymers enhance performance. The instant phosphonates, when added to waters containing silica and hardness at a pH of about 9.0, prevent the formation of silicates and their deposition. The instant method has been shown to effectively maintain up to 300 mg/L SiO.sub.2 and greater than 200 mg/L hardness without substantial deposition on heat exchanger surfaces.